1. Field of the Invention
This invention relates to a method for installing an undersea metal pipe rising in a catenary, especially metal pipes which are used to deliver oil and gas originating from undersea wells and/or gathering lines to a floating unit.
2. Description of the Related Art
As a result of the growing demand for oil and gas, and due to the natural exhaustion of conventional producing regions, offshore oil and gas prospecting and production has increased greatly in recent years.
New hydrocarbon producing provinces have appeared as a consequence of this increase in prospecting work. Initially, platforms fixed on the sea floor were used to receive the output from producing wells and perform primary processing, and output was generally discharged to gathering stations located on shore, relatively close to the producing region, using rigid metal pipes.
Connection between producing wells and platforms was generally by means of special flexible pipes, of relatively high cost. More recently, as a way of reducing costs, there has been a change to the use of rigid metal pipes to deliver the output from wells to primary treatment units located on fixed platforms.
With wells located at increasingly greater water depths (over 600 m) coming into production, it has been necessary to use floating units to accommodate the primary treatment units for the oil and gas produced.
This has created a problem with the use of rigid metal pipes for the delivery of output, because floating units undergo constant movement at the surface, which makes it difficult to use such rigid metal pipes. A change has therefore been made to the use of a hybrid system, in which the greater length of the production flow line extending from the well to a specific point close to a floating unit comprises rigid metal piping, of lower cost, and a section rising from the seabed to the floating unit (known to specialists as a xe2x80x9cRISERxe2x80x9d) comprising special flexible pipe of higher cost.
Various methods are known for installing rigid metal pipe on the seabed. Of these, a method characterized in that sections of rigid metal pipe are welded together continuously on an onshore base, with the pipe obtained in this way being wound onto a large diameter drum, deforming plastically, has become prominent. This method of winding is known to specialists as xe2x80x9cREELINGxe2x80x9d.
Long lengths of pipe which are produced on shore at a very low cost are obtained in this way, thus avoiding having to make most of the special welds for connecting the metal pipe on the deck of the launching vessel, thus reducing the time for which these vessels are used and consequently reducing the final cost.
During the operation of launching pipe wound on drums the curvature of the pipe is reversed through the use of straighteners located in a pipe handling unit appropriate for this purpose, which is normally located at the stern of the vessel.
This method has proved to be the most economical and most efficient for installing rigid metal pipe on the seabed.
In more recent years, after intense technological research and development, use of rigid metal pipes to form the ascending length (riser) connecting the length of pipe on the seabed to the floating unit has come to be considered as a replacement for special flexible pipes. This length, which would comprise a rigid metal pipe in a catenary configuration, is known by the abbreviation SCR (STEEL CATENARY RISER).
The rigid ascending length (RISER) of steel has some advantages over a flexible riser, because of its lower material cost, and also because its installation constitutes a natural extension of the undersea pipe located on the seabed. In principle, the same vessel which is responsible for installing the rigid metal pipe on the seabed could install a rigid catenary riser.
However, this gives rise to a problem. The rigid catenary riser is subject to severe cyclical loads due to the movements of the floating unit, and as a consequence fatigue resistance becomes a fundamental property if a rigid catenary riser is to be able to offer problem-free operation, particularly at the welded joints.
In this respect use of the reeling method has proved unsuitable, because the plastic deformation which occurs in the undersea metal pipe material can give rise to serious consequences, in terms of stress concentration and the growth of defects which would be acceptable for undersea metal pipes supported on the seabed but unacceptable for rigid catenary risers.
To overcome this problem a method known by specialists as xe2x80x9cJ-layxe2x80x9d is used, and this basically comprises welding metal pipes to each other on the launching vessel itself at the time when they are launched. The pipe is launched in a position which is close to vertical, which guarantees that it is installed within the elastic limit, thus avoiding occurrence of the abovementioned problems relating to the reeling method.
Although the J-lay method is a good solution, it has the disadvantage of being quite slow, and, as a consequence, has a high cost, due to prolonged use of the pipe launching vessel.
Therefore, when an undersea metal pipe which comprises a section supported on the ocean bed and a section rising as a catenary has to be installed it is necessary to use two separate vessels, one which uses the reeling method to install the section supported on the ocean bed, and a second vessel using the J-lay method to install the catenary length. This considerably increases the total cost of installing undersea metal pipe.
New solutions therefore have to be developed for launching rigid metal catenary risers, and these must offer high performance and a low operating cost. This invention presents a solution which addresses these two requirements.
This invention relates to a method for installing an undersea metal catenary riser pipe, which in general terms comprises:
initially launching, by means of a launching vessel, a length of metal pipe which will be supported on the ocean bed, the said length of metal pipe previously welded and wound onto a recipient device, being unrolled from the recipient device and passing through a straightener pulled by a puller located on a launching ramp on the said launching vessel before being laid on the seabed,
the method additionally comprising the following stages:
a) when the said launching vessel is prepared to perform the final operation of laying the length of metal pipe on the seabed, a length of previously welded metal pipe of a length equivalent to the length required for an intermediate stretch of undersea metal catenary riser pipe has to be wound onto the innermost layer of the recipient device, and the remainder of the metal pipe which will be laid on the seabed has to be wound as the outer layer,
b) after the final length of metal pipe has been launched, the end which will be connected to the catenary riser is supported by a supporting bracket,
c) a first section is then fitted in the said launching ramp and coupled to the said end of the metal pipe which is connected to the catenary riser suspended by a supporting bracket,
d) the operations of joining, pre-heating, welding, weld inspection and coating of the said section are then performed at a working station located between the puller and the supporting bracket,
e) the supporting bracket is then operated so that it momentarily leaves the suspended metal pipe unsupported, and the latter is then suspended by means of a supporting and lowering system connected to the upper end of the said section,
f) the support and lowering system is then operated in order to enable the section which has just been welded to be launched together with the metal pipe until its upper end is located at the said workstation, in such a way that a new section can be welded to it,
g) the said supporting bracket is then operated to compress the recently fitted section, supporting the weight of the entire suspended metal pipe and the recently fitted section,
h) the said support and lowering system is then disconnected from the upper part of the recently fitted section so that the next section which is to be fitted can be handled,
i) the above stages referred to by c, d, e, f, g, h are repeated in such a way that a sufficient number of sections are coupled together in order to obtain the previously determined length of the section of pipe which will form the length of the catenary riser pipe which will be in contact with the seabed,
j) after the last section in the length in contact with the seabed has been welded, one end of the previously welded length of metal pipe of a length equivalent to the length required for an intermediate length of the catenary riser pipe which has previously been wound as the innermost layer on the recipient device and which forms the said intermediate length is then welded to the upper end of the last section,
k) from this point the intermediate length of the catenary riser pipe is unwound from the recipient device and passed through a straightener, pulled by a puller, located on a launching ramp on the said launching vessel, before being launched,
I) after all the intermediate length of the catenary riser has been launched it will be necessary to launch the final critical length of the top of the catenary riser pipe, repeating the stages referred to above by c, d, e, f, g, h until the length of the top stretch of the catenary riser pipe has been completed.
Accordingly, the present invention provides a method of attaching a first pipe section to a pre-reeled pipe, the method comprising:
unreeling said pre-reeled pipe so as to launch it in the sea from a vessel;
supporting in a first support located on said vessel said unreeled pipe near a top end thereof;
attaching said first pipe section to the end of said supported pipe using attaching means on said vessel.
Further, the invention provides a method of attaching a pre-reeled pipe to a pipe section, the method comprising:
supporting in a first support located on a vessel said pipe section near a top end thereof;
unreeling part of said pre-reeled pipe on said vessel;
attaching one end of said pre-reeled pipe to the end of said supported pipe section using attaching means on said vessel.
Furthermore, the invention provides a vessel for launching two types of piping which together constitute an underwater pipe system, said vessel comprising:
a reel for holding at least one length of coiled piping;
guiding/retaining apparatus comprising:
a pipe straightener for straightening the coiled piping as it is unreeled;
a puller for urging the pipe in a substantially downward direction;
a support for holding the pipe and supporting the pipe""s weight;
said vessel further comprising a launching ramp for presenting discrete straight pipe sections to said guiding/retaining apparatus;
a working station providing the apparatus necessary to connect separate pipe sections together.
In addition, the invention provides an undersea pipe system comprising:
a section of pipe on the sea bed having a characteristic stress profile which has been formed from a coiling/straightening process;
a first series of pipe sections connected to said sea bed section, said first series of pipe sections not having the characteristic stress profile typical of coiled and straightened pipe sections;
a further section of coiled and straightened piping connected to said first series of pipe sections;
a second series of pipe sections connected to said further section of coiled and straightened piping, said second series of pipe sections not having said characteristic stress profile;
wherein said second series of pipe sections is connected to a floating unit.